DE102010007270B3 - Mold core for shaping and tempering a hollow structure - Google Patents

Abstract

A mold core (10, 20) for molding and tempering a hollow structure (11), comprising: an electrically non-conductive inner area (13, 23) and an electrically conductive outer area (12, 22); as well as two externally accessible electrical contacts for applying an electrical voltage, the thickness of the outer area (12, 22) varying in a targeted manner.

Description

The present invention relates to a mold core for shaping and tempering a hollow structure and to a method for producing such a mold core.

Shaped cores are usually used for the production of hollow structures, such as fiber composite structures. It can be differentiated between lost cores and permanent cores, whereby permanent cores are reusable, while lost cores are washed out of the finished hollow structure after a single use and thereby destroyed. This can be useful, for example, if due to the geometry of the hollow structure to be produced there is no possibility of an ordinary removal of the mandrel. Lost cores can be chemically, thermally or by means of a liquid solvable, that is rinsable, and usually consist of a mold core base material and a binder.

From the EP 1 323 686 B1 shows a method for the production of mold cores, which in turn are to be used for forming hollow bodies of fiber-reinforced ceramic materials. By electrical resistance heating of the mandrel is heated. For this purpose, the starting material of the mold core electrically conductive additives are mixed homogeneously. By means of such mandrels in particular local overheating should be avoided. However, this is not sufficiently possible in practice.

The present invention is therefore based on the object to provide a mandrel which is heated so that targeted arbitrarily specifiable temperature distribution results on its outer surface.

According to the invention this object is achieved by a mold core for shaping and tempering a hollow structure, wherein the mold core has an electrically non-conductive inner region and an electrically conductive outer region and two externally accessible electrical contacts for applying an electrical voltage, wherein the thickness of the outer region deliberately varied. Instead of the term "strength", the term "thickness" can also be used. In addition, the term "electrically conductive" can also be used instead of the term "electrically conductive".

In particular, the outer portion of the mandrel may be formed of a mandrel base material enriched with electrically conductive material such as carbon black, graphite, short and / or long carbon fibers and / or metal powder or fibers, the proportion of electrically conductive material being constant is or varies deliberately.

According to another embodiment, a contact surface between the inner region and the outer region is smooth. "Smooth" should be interpreted here as a low roughness of the contact surface.

Alternatively, a contact surface between the inner region and the outer region is rib-shaped.

According to a particular embodiment, a contact surface between the inner region and the outer region is coated with silver paint.

According to a further embodiment, the mandrel is formed as a lost core and can be flushed out using a liquid, for example.

The invention further relates to a method for producing a mandrel comprising forming a first body for forming an inner electrically non-conductive region of the mandrel; and applying molding material to the first body to form an outer electrically conductive portion of the mandrel and attaching two externally accessible electrical contacts to apply an electrical voltage, wherein the thickness of the outer region selectively varies. The strength can vary spatially or area specifically.

The method claims relate to advantageous embodiments of the invention

A further aspect of the invention relates to the use of a mold core as described above for shaping and tempering a hollow structure, comprising: introducing a material intended for producing the hollow structure into a mold; Inserting the mold core into the mold; Closing the mold and making an electrical connection for applying an electrical voltage to the two electrical contacts of the mandrel for heating the material; and after completion of the curing of the material to the hollow structure, releasing the electrical connection and removing the mold core, preferably by rinsing out the hollow structure and the mold. Alternatively, the mold core can only then be removed from the hollow structure after it has already been removed from the mold.

The invention is based on the surprising finding that a multilayer structure of a mold core due to the formation of an inner and an outer region and an interaction of the thickness of the outer region with the thickness of the inner region can influence and control the heat generation properties of a flowing stream in this way, that at any point, in particular at any point of the outer surface of the mandrel, a certain amount of heat can be achieved. The local temperature to be generated can be specified more precisely than is possible with any other known production method or any other known mold core with comparatively little effort. Also, the mold core according to the invention allows a particularly efficient heating of a hollow structure, since the heat arises directly at the surface of the core, where it is needed.

A particularly surprising effect of the present invention is that the construction of two areas not only allows a control of the heat distribution, but also provides a thermal insulation of the inner region and thus avoids a heat sink inside the mold core. At the same time there is no unnecessary heating of areas of the mold core, which are not in contact with the hollow structure, so that is an additional advantage to call an efficient use of energy. This also offers over the use of liquid or other movable heat transfer the particular advantage that the mandrel is particularly simple both in terms of its structure and the process steps that are necessary for the construction.

A further advantage is that the particular difficulties posed by mobile heat transfer media associated with soluble lost cores are avoided.

Another particular advantage is that alternative heating methods, which provide for instance metallic heating coils in the mandrel, are difficult to reconcile with the consistency and in particular the dissolution of a lost core.

Moreover, the present invention allows to avoid the formation of thermal local expansions and thus stresses in the mold core. As has been shown above in the discussion of the known technique, thermal local expansions can not be effectively counteracted so that prior art stresses based thereon also pose an unsolved problem.

Further features and advantages of the invention will become apparent from the claims and from the following description in which exemplary embodiments are explained in detail with reference to the schematic drawings, in which:

1 a side view of a mandrel according to a particular embodiment of the invention in a hollow structure in section; and

2 a side view of a mold core according to another embodiment of the invention shows.

The in 1 shown mold core 10 is in surface contact with a hollow structure 11 or its starting material in a mold (not shown) which is to be shaped and tempered by the mandrel. The mold core 10 has an inner area 13 and an outer area 12 on, with the outer area 12 the inner area 13 covered on all sides towards the hollow structure. The mold core 10 may in the embodiment shown in sections of different thickness of the mandrel 10 , ie cylindrical sections with different radii, are subdivided. Such is the mold core 10 in the section 19a considerably thinner than in the section 19b ,

The outer area 12 is preferably made of molding sand, which is enriched with certain electrically conductive substances in order to increase its electrical conductivity. Such a substance may be, for example Leitruß or graphite. Alternatively, however, short or long carbon fibers are also suitable; alternatively, metal powder and / or metal fibers can also be used. Also the inner area 13 is preferably made of molding sand and thus does not conduct the electrical current or only a small extent. Also, the inner area points 13 preferably only a low thermal conductivity. From the 1 In particular, it can be seen that the thickness in the radial direction of the outer region 12 opposite to the strength of the inner area 13 may vary locally. So his strength is for example in the section 19a considerably larger than in the section 19b , Due to a greater strength results in a lower electrical resistance, which in turn leads to a lower heating effect of a current flowing through it.

According to a particular embodiment, the mold core 10 be designed as a hollow core, wherein the inner region 13 at least partially has a cavity.

For electrical resistance heating of the mandrel is the outer region 12 provided with contacts to which an electrical voltage can be applied. The contacts are in 1 Not shown. You can, however, anywhere on the outer area 12 to be appropriate. For example, the contacts may be mounted on opposite sides of the mold core; However, depending on the requirements of the hollow structure to be cured, a different constellation of the contacts is conceivable.

To the different thermal conductivities of the inner area 13 and the outer area 12 To take into account, the contact surface between the two areas can be smooth. A smooth surface is understood to mean an area of low roughness. Alternatively, however, the contact surface may also be rib-shaped or otherwise have a three-dimensional structure. In particular, in this way the contact surface between the inner region 13 and the outer area 12 be enlarged. According to a particular embodiment, the surface can be enlarged by rib-shaped contacts, for example by a factor of 3, 4 compared to a smooth contact surface. Alternatively, the contact surface may be provided with a silver paint coating. This serves in particular a further thermal insulation of the inner region 13 from the outer layer 12 in order to be able to adjust the desired heat distribution even more precisely.

The shape of the mandrel according to the invention can be of any desired design both in the longitudinal and in the transverse direction.

In the following, a particular embodiment will be described, in which a hollow structure is to be produced, which has a cavity in the form of two coaxially juxtaposed hollow cylinder with different radii. Given the boundary condition that the mold core corresponding to the cavity should produce uniform heat development over its entire outer surface, the required cross-sectional area of the inner area at a given section should be determined given the outer dimensions of the mold core. May the outer area 12 of the mold core in the section 19a an inner radius r _{inside, 1} and an outer radius r _{outside, 1} and in the section 19b have an outer radius r _{outside, 2} . It then raises the question of how large the inner radius r _{inside, 2} in the section 19b must be to deliver the same heat output to the outside. For this we take the following considerations:
The circumference of a cylinder is as U = 2π · r.

As the circumference U increases, the heat output to be provided increases linearly, which is necessary in order to effect a constant temperature control. This heat output is also proportional to the electrical resistance R. Further, the cross-sectional area of a cylindrical inner region 13 , A _inside = π · r ² _inside , the total cross-sectional area A of a cylinder i is A _i = π · r ² _outside .

angegeben werden. Ferner gilt

This allows the cross-sectional area A _{outside, i of} the outer area 12 without the inner area 13 for a cylinder i as A _{outside, i} = A - A _inside specify. The electrical resistance of a cylinder i is inversely proportional to the cross-sectional area A _{outside, i} its outer region 12 , For two cylinders 1 and 2, this relationship can be considered

be specified. Furthermore, applies

berechnet werden. Anders ausgedrückt kann also bei festem Radius r_außen,1 an einem ersten Abschnitt 19a des Formkerns 10 und festem Radius r_außen,2 an einem zweiten Abschnitt 19b des Formkerns 10 der Außenradius (= r_innen,1) des inneren Bereichs 13 am ersten Abschnitt 19a so gewählt werden, dass am ersten Abschnitt 19a eine vorbestimmte Erwärmung erzeugt wird; und der Außenradius (= r_innen,2) des inneren Bereichs 13 am zweiten Abschnitt 19b abhängig von r_innen,1, r_außen,1 und r_außen,2 so gewählt werden, dass am zweiten Abschnitt 19b die gleiche Erwärmung wie am ersten Abschnitt 19a erzeugt wird. Damit ist die Querschnittsfläche des inneren und des äußeren Bereichs am Abschnitt 19b bekannt.From this equation, the searched radius r _{inside, 2} out

be calculated. In other words, therefore, at fixed radius r _{outside, 1} at a first section 19a of the mold core 10 and fixed radius r _{outside, 2} at a second portion 19b of the mold core 10 the outer radius (= r _{in, 1} ) of the inner area 13 on the first section 19a be chosen so that on the first section 19a a predetermined heating is generated; and the outer radius (= r _{in, 2} ) of the inner area 13 on the second section 19b depending on r _{inside, 1} , r _{outside, 1} and r _{outside, 2} are chosen so that on the second section 19b the same warming as on the first section 19a is produced. Thus, the cross-sectional area of the inner and outer regions is at the section 19b known.

In an alternative embodiment with square, rectangular or even any cross-sectional shapes of the mandrel, the above equations are analogously applicable, wherein only the correct functions for calculating the respective cross-sectional area and the circumference are to be used.

The 2 shows a further embodiment of the present invention, wherein a mold core 20 with an inner area 23 and an outer area 22 is shown. The areas 22 and 23 point in the longitudinal direction of the mandrel 20 a cross section that changes continuously over several sections, such as in the marked A section. Transverse to the longitudinal direction of the mandrel, the areas 22 and 23 a rectangular cross section. 2 also shows a temperature profile 21 on the surface of the outer area 22 , This shows, for example, that in section A a much lower temperature prevails than in section B. This difference is based on the fact that the strength of the outer region 22 much smaller in section B than in section A; This indicates the outer area 22 in section B, a greater electrical resistance than in section A, which leads to a higher heating effect and thus to an elevated temperature when flowing through an electric current. The temperature profile 21 also shows a slight increase in the temperature in section A of the outer area 22 in the direction of section B. This results from the tapered thickness of the outer area 22 in section A in the direction of section B and is based on a concomitant continuous reduction of the electrical resistance of the outer region 22 , The texture of the areas 22 and 23 of the 2 as well as the other properties of the mold core shown 20 Incidentally, the same properties as the mold core 10 out 1 exhibit.

The above-described approaches for influencing a local heating within a mold core, which as shown particularly based on an adjustment of the area strengths, can be advantageously combined with a spatially or area varying degrees of enrichment of additives in the outer area 12 , In particular, a particularly strong local variation of the area intensities required by the above provision can be attenuated by adapting the electrical conduction properties by adding electrically conductive additives in the relevant area such that only a smaller variation of the area intensities is necessary. In the selection of such additives is not only to electrical conductivity, but also pay attention to thermal conductivity, for example, with an increase in the electrical resistance and an improvement in the thermal conductivity should be sought. This requirement is met for example by Leitruß, graphite or carbon fibers, the latter may be in the form of short or long fibers. Alternatively or additionally, metal powder and / or metal fibers can also be used. Finally, combinations of said materials are usable. Preferably, however, graphite is used, since this substance not only has a high thermal conductivity and electrical conductivity, but also has a high temperature resistance and in particular a high thermal shock resistance. The latter promotes acceleration of heating and cooling phases during the curing process of a fiber composite structure. In addition, graphite has a high resistance to oxidation and is particularly resistant to certain chemicals. Graphite can also be produced in high purity and is easy to process, finally also offers good environmental compatibility and is harmless to health in the processing.

The invention described above can be used for the production of hollow structures, for example fiber composite structures, as well as in die-casting or in the injection molding process. In general, the invention represents an improvement in the production of complex hollow structures, in which a temperature of the mold is required or advantageous.

To produce a mandrel according to the embodiments described above, essentially two steps are necessary. In a first step, the inner electrically or only slightly conductive region is formed, for example, a printing process is used. Typically, the inner region consists of molding sand. In a second step, the outer electrically conductive region 12 on the inner area 13 applied, leaving the outer area 12 the inner area 13 covered. Further, contacts are made to the outer area 12 applied to an electrical voltage to the outer area 12 to create. The strengths of the two areas are the respective ones. According to requirements of the application, in particular taking into account the above embodiments of the invention.

To the mold core according to the invention for shaping and tempering a hollow structure to be cured 11 to use, according to a particular embodiment, first a material provided for producing the hollow structure is introduced into a mold. Subsequently, the mold core is embedded in the mold, so that it forms a cavity together with this. For this purpose, any known to the expert procedure can be selected. Subsequently, an electrical voltage is applied to the two contacts of the mold core 10 created to heat the material. After completion of the curing of the hollow structure 11 the electrical connection is released and the mold core 10 - If it is a lost core - rinsed out of the hollow structure and the mold or only after removal of the hollow structure from the mold or - if the mold core is designed as a permanent core - in other ways from the hollow structure 11 and removed from the mold for later reuse.

In the embodiments presented above, a mold core is described which represents a novel and particularly efficient way of achieving a certain distribution of for curing a surrounding fiber composite structure. In addition to the already mentioned advantages of the invention should be pointed to a particularly simple, fast and inexpensive core production, which is also advantageous to automate. Due to the simple design, low costs are to be expected in the construction of such a mold core, in particular since the main components of a corresponding system comprise only one current generator and one current regulator. The already mentioned good environmental compatibility of the mold core according to the invention is further based on the fact that no medium for the heat input into the core is necessary. It is thus not a potentially wasteful waste product to be disposed of.

The features of the invention disclosed in the present description, in the drawings and in the claims may be essential both individually and in any desired combinations for the realization of the invention in its various embodiments.

LIST OF REFERENCE NUMBERS

10, 20

mold core

11

hollow structure

12, 22

outer area

13, 23

inner area

18, 28

contact area

19a

section

19b

section

21

temperature profile

_{inside, 1}

Inner radius of the outer area

_{inside, 2}

Inner radius of the outer area

_{outside, 1}

Outer radius from the outer area

_{outside, 2}

Outer radius from the outer area

Claims (13)

Mold core ( 10 . 20 ) for shaping and tempering a hollow structure ( 11 ), comprising: an electrically non-conductive inner region ( 13 . 23 ) and an electrically conductive outer region ( 12 . 22 ); and two externally accessible electrical contacts for applying an electrical voltage, wherein the strength of the outer region ( 12 . 22 ) varies in a targeted manner. Mold core ( 10 . 20 ) according to claim 1, characterized in that the outer area ( 12 . 22 ) consists of a mold core base material which is enriched with electrically conductive material, such as Leitruß, graphite, short and / or long carbon fibers and / or metal powder or fibers, wherein the proportion of the electrically conductive material is constant or varies selectively. Mold core ( 10 . 20 ) according to claim 1 or 2, characterized in that a contact surface ( 18 . 28 ) between the inner area ( 13 . 23 ) and the outer area ( 12 . 22 ) is smooth. Mold core ( 10 . 20 ) according to one of the preceding claims, characterized in that the contact surface ( 18 . 28 ) between the inner area ( 13 . 23 ) and the outer area ( 12 . 22 ) is formed rib-shaped. Mold core ( 10 . 20 ) according to one of the preceding claims, characterized in that a contact surface ( 18 . 28 ) between the inner area ( 13 . 23 ) and the outer area ( 12 . 22 ) is coated with silver paint. Mold core ( 10 . 20 ) according to one of the preceding claims, characterized in that the mandrel ( 10 . 20 ) is formed as a lost core and is preferably rinsed out using a liquid. Method for producing a mold core ( 10 . 20 ), comprising: forming a first body to form an inner electrically non-conductive region ( 13 . 23 ) of the mandrel ( 10 . 20 ); and applying molding material to the first body to form an outer electrically conductive region ( 12 . 22 ) of the mandrel ( 10 . 20 ) and attaching two externally accessible electrical contacts for applying an electrical voltage, wherein the strength of the outer region ( 12 . 22 ) varies in a targeted manner. Method according to claim 7, characterized in that the outer region ( 12 . 22 ) is formed from a mold core base material which is enriched with electrically conductive material, such as Leitruß, graphite, short and / or long carbon fibers and / or metal powder or fibers, wherein the proportion of the electrically conductive material is constant or varies selectively. Method according to claim 7 or 8, characterized in that the contact surface ( 18 . 28 ) between the inner area ( 13 . 23 ) and the outer area ( 12 . 22 ) is formed smooth. Method according to one of claims 7 to 9, characterized in that the contact surface ( 18 . 28 ) between the inner area ( 13 . 23 ) and the outer area ( 12 . 22 ) is formed rib-shaped. Method according to one of claims 7 to 10, characterized in that the contact surface ( 18 . 28 ) between the outer area ( 12 . 22 ) and the inner area ( 13 . 23 ) is coated with silver paint. Method according to one of claims 7 to 11, characterized in that the application of the outer area ( 12 . 22 ) on the inner area ( 13 . 23 ) takes place as baking under pressure and / or heat. Use of a mold core ( 10 . 20 ) according to one of claims 1 to 6 for shaping and tempering a hollow structure ( 11 ) comprising: introducing one for producing the hollow structure ( 11 ) provided material in a mold; Introduction of the mold core ( 10 . 20 ) into the mold; Closing the mold and establishing an electrical connection for applying an electrical voltage to the two electrical contacts of the mold core ( 10 . 20 ) for heating the material; and after complete hardening of the material to the hollow structure ( 11 ) Releasing the electrical connection and rinsing the mold core ( 10 . 20 ) from the hollow structure ( 11 ) and the shape.

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